Effects of the El Niño-Southern Oscillation in the United States

This article describes the Effects of ENSO in the United States. The El Niño-Southern Oscillation affects the location of the jet stream, which alters rainfall patterns across the West, Midwest, the Southeast, and throughout the tropics. The shift in the jet stream also leads to shifts in the occurrence of severe weather, and the number of tropical cyclones expected within the tropics in the Atlantic and Pacific oceans affected by changes in the ocean temperature and the subtropical jet stream.

Temperature

During El Niño, the northern tier of the lower 48, as well as southern Alaska, exhibits above normal temperatures during the fall and winter, while the Gulf coast experiences below normal temperatures during the winter season. [Climate Prediction Center. [http://www.cpc.ncep.noaa.gov/products/predictions/threats2/enso/elnino/UStrank/ond.gifAverage October-December (3-month) Temperature Rankings During ENSO Events.] Retrieved on 2008-04-16.] [Climate Prediction Center. [http://www.cpc.ncep.noaa.gov/products/predictions/threats2/enso/elnino/UStrank/djf.gifAverage December-February (3-month) Temperature Rankings During ENSO Events.] Retrieved on 2008-04-16.]

Precipitation

Across Alaska, La Niña events lead to drier than normal conditions, while El Niño events do not have a correlation towards dry or wet conditions. During El Niño events, increased precipitation is expected in California due to a more southerly, zonal, storm track. [John Monteverdi and Jan Null. [http://tornado.sfsu.edu/geosciences/elnino/elnino.html WESTERN REGION TECHNICAL ATTACHMENT NO. 97-37 NOVEMBER 21, 1997: El Niño and California Precipitation.] Retrieved on 2008-02-28.] During La Niña, increased precipitation is diverted into the Pacific Northwest due to a more northerly storm track. [Nathan Mantua. [http://www.ccb.ucar.edu/lanina/report/mantua.html La Niña Impacts in the Pacific Northwest.] Retrieved on 2008-02-29.] During La Niña events, the storm track shifts far enough northward to bring wetter than normal conditions (in the form of increased snowfall) to the Midwestern states, as well as hot and dry summers. [Southeast Climate Consortium. [http://www.agclimate.org/Development/apps/agClimate/controller/perl/agClimate.pl/agClimate.pl?function=climforecast/outlook.html&location=local&type SECC Winter Climate Outlook.] Retrieved on 2008-02-29.] [Reuters. [http://www.reuters.com/article/domesticNews/idUSN1619766420070216 La Nina could mean dry summer in Midwest and Plains.] Retrieved on 2008-02-29.] During the El Niño portion of ENSO, increased precipitation falls along the Gulf coast and Southeast due to a stronger than normal, and more southerly, polar jet stream. [Climate Prediction Center. [http://www.cpc.noaa.gov/products/analysis_monitoring/ensocycle/ensorain.shtml El Niño (ENSO) Related Rainfall Patterns Over the Tropical Pacific.] Retrieved on 2008-02-28.] In the late winter and spring during El Niño events, drier than average conditions can be expected in Hawaii. [Pao-Shin Chu. [http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0442(1995)008%3C1697%3AHRAAEN%3E2.0.CO%3B2 Hawaii Rainfall Anomalies and El Niño.] Retrieved on 2008-03-19.] On Guam during El Niño years, dry season precipitation averages below normal. However, the threat of a tropical cyclone is over triple what is normal during El Niño years, so extreme shorter duration rainfall events are possible. [Pacific ENSO Applications Climate Center. [http://www.soest.hawaii.edu/MET/Enso/peu/2006_4th/guam_cnmi.htm Pacific ENSO Update: 4th Quarter, 2006. Vol. 12 No. 4.] Retrieved on 2008-03-19.] On American Samoa during El Niño events, precipitation averages about 10 percent above normal, while La Niña events lead to precipitation amounts which average close to 10 percent below normal. [Pacific ENSO Applications Climate Center. [http://www.soest.hawaii.edu/MET/Enso/enso/rain/asamoa.html RAINFALL VARIATIONS DURING ENSO.] Retrieved on 2008-03-19.] Despite known changes in tropical cyclone activity due to changes in the El Niño/Southern Oscillation (ENSO) in the Atlantic basin, [Dr. David Hodell and Ray G. Thomas. [http://ess.geology.ufl.edu/usra_esse/ENSO_Impacts.html Impacts of ENSO.] Retrieved on 2008-03-19.] there is no relationship between rainfall in Puerto Rico and the ENSO cycle.

nowfall

During an El Niño, snowfall is greater than average across the southern Rockies and Sierra Nevada mountain range, and is well-below normal across the Upper Midwest and Great Lakes states. During a La Niña, snowfall is above normal across the Pacific Northwest and western Great Lakes. [Climate Prediction Center. [http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/us_impacts/ustp_impacts.shtml ENSO Impacts on United States Winter Precipitation and Temperature.] Retrieved on 2008-04-16.]

evere weather

During El Niño, the jet stream is oriented from west to east across the southern portion of the United States, making the region more susceptible to severe weather outbreaks. During La Niña, the jet stream and severe weather is likely to be farther north than normal. [Climate Prediction Center. [http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#TORNADOES What impacts do El Niño and La Niña have on tornado activity across the country?] Retrieved on 2008-04-16.]

Tropical cyclones

Due to changes in upper level winds caused by variations in the ENSO cycle, the likelihood of an Atlantic hurricane hitting the Continental United States is increased during La Niña conditions, and decreased during El Niño conditions.cite web|url=http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#HURRICANES|publisher=Climate Prediction Center|format=FAQ|accessdate=2008-03-21|title=How do El Niño and La Nina influence the Atlantic and Pacific hurricane seasons?]

References